Sizing an RTD installation - Part 2
By Michel Adar on Nov 15, 2009
Now that we have the expected throughput in terms of the number of requests per second, lets look at other sizing factors.
Response time - sometimes the volume of requests is not smoothly distributed and there may be peaks of requests coming at the same time. If there are strict response time requirements, like having an average below 30ms with a maximum of 60ms for 99% of the requests, then we need to consider the maximum number of requests that are going to be processed in parallel. To achieve the highest performance for the highest number of requests we will design for between 3 and 6 requests being processed in parallel per CPU core or hardware hyperthread.
Session Initializations - when a session is initialized there are a few extra things that happen when compared with requests that come after initialization. First, depending on whether the RTD server manages session affinity, a new entry is created in the sessions table, which typically requires at least one database write. Additionally, the in memory session is typically filled from the configured data sources. The speed of these operations is totally driven by the performance of the source databases. If an application has many more session initialization than other types of messages, then the throughput may be affected even though the total number of requests is not too high for the configuration.
Single Point of Failure and High Availability - in most cases the system is configured to provide High Availability (HA) and resiliency to server failure or lack of availability (for rolling maintenance for example). RTD is typically configured with a number of servers to avoid the single point of failure. Sometimes it is also configured with multiple sites for HA and Disaster Recovery (DR). In this context it is important to consider the option of relying on default responses to cope with outages of the RTD servers. I know of one RTD server that has been working since 2005 and has been down for maintenance only for a few hours total since it started.
In the next entry we will finally talk about the sizing of the servers.